Appendix B: Quick References—Medically Important Bacteria
| Organism | Characteristics | Diseases | Habitat/Pathogenesis | Pathogenesis | Laboratory Diagnosis | Treatment/Prevention |
|---|---|---|---|---|---|---|
| Gram positive cocci | ||||||
| Staphylococcus aureus | Gram positive cocci in clusters.
Coagulase positive. Most isolates produce lactamase. |
Abscesses of many organs; skin and soft tissue infections/ impetigo, endocarditis, osteomyelitis, septic arthritis, sepsis and wound site infections. Also, hospital acquired pneumonia. Also, exotoxin-mediated diseases such as gastroenteritis (food poisoning), toxic shock syndrome, and scalded skin syndrome. Methicillin resistant Staphylococcus aureus (MRSA) is the most common cause of skin abscesses, pneumonia, necrotizing fasciitis, and sepsis in immunocompetent patients and users of intravenous drugs. |
Main habitat is human nose; also found on human skin.
Transmission is via hands. |
Abscess containing pus; pyogenic lesions that enter bloodstream; Predisposing factors: suture, skin breaks, IV drug use.
Endotoxins: Enterotoxin, which causes food poisoning, is also a superantigen. Food poisoning has a short incubation period because it is preformed in food. Scalded skin syndrome toxin is a protease that cleaves desmoglein in tight junctions in the skin. Protein A binds to the heavy chain of IgG. Reduces phagocytosis. Gamma heavy chain cannot bind to its receptor on the surface of neutrophils and macrophages. |
Gram stained, smear and culture.
Blood agar: Yellow or gold colonies |
Penicillin G for sensitive isolates --> β-lactamase --> Vancomycin Vancomycin-resistant strains exist. Cefazolin is used to prevent surgical wound infections. No vaccine. Handwashing reduces transmission. |
| Staphylococcus epidermidis | Gram positive cocci in clusters. Coagulase negative. Catalase positive. |
Endocarditis on prosthetic heart valves, prosthetic hip infection, intravascular catheter infection, cerebrospinal fluid shunt infection, neonatal sepsis. | Normal flora of the human skin and mucous membranes.
Patient’s own strains cause infection, but transmission from person to person via hands may occur. |
Glycocalyx producing strains adhere well to foreign bodies such as prosthetic implants and catheters.
Low virulence organism that causes disease in immunocompromised patients and in those with implants. It is a major cause of hospital acquired infections. No exotoxins have been identified. |
Gram stained smear Blood agar: Whitish, nonhemolytic colonies Coagulase negative S. epidermidis is sensitive to novobiocin, whereas the other coagulase negative Staphylococcus, Staphylococcus saprophyticus, is resistant |
Vancomycin plus either rifampin or an aminoglycoside.
It produces β-lactamases No vaccine. |
| Streptococcus pyogenes (Group A Streptococcus) | Gram positive cocci in chains. β-Hemolytic colonies. Catalase negative. Bacitracin sensitive. β- Hemolytic streptococci are subdivided into group A, B, etc., by differences in the antigenicity of their cell wall carbohydrate. |
Pharyngitis and cellulitis; toxigenic diseases, for example necrotizing fasciitis (“flesheating” streptococci) and scarlet fever; immunologic (antibodymediated) diseases (e.g., rheumatic fever and acute glomerulonephritis). |
Human throat and skin.
Transmission is via respiratory droplets. |
Suppurative Diseases: Hyaluronidase (“spreading factor”) mediates subcutaneous spread seen in cellulitis. Toxigenic infections: Immunologic diseases: M protein reacts with myosin in cardiac muscle, and acute glomerulonephritis is caused by immune complexes formed between streptococcal antigens and antibodies to those antigens. The immune complexes are trapped by glomeruli, complement is activated, neutrophils are attracted to the site by C5a, and proteases produced by neutrophils damage glomeruli. |
Suppurative infections: Gram stained smear and culture. Blood agar: β-Hemolytic colonies (Hemolysis due to streptolysins O and S.) If isolate is sensitive to bacitracin, it is identified as S. pyogenes. ELISA tests for group A streptococcal antigens in throat swabs. Immunological disease: If acute glomerulonephritis is suspected, antibody to streptococcal DNase B is used as evidence of a previous skin infection by S. pyogenes. |
Penicillin G (no significant resistance).
Oral penicillin V or amoxicillin is often used. Prevention—Penicillin is used in patients with rheumatic fever to prevent recurrent S. pyogenes pharyngitis. This prevents additional damage to heart valves. No vaccine. |
| Streptococcus agalactiae (Group B Streptococcus) | Gram positive cocci in chains. β-Hemolytic colonies subdivided into group A, B, etc., by differences in the antigenicity of their cell wall carbohydrate. Catalase negative. |
Neonatal meningitis and sepsis. | Human vagina.
Transmission occurs during birth. |
Pyogenic organism.
No exotoxins identified. Predisposing factors to neonatal infection include rupture of membranes more than 18 hours before delivery, labor prior to 37 weeks (infant is premature), absence of maternal antibody, and heavy colonization of the genital tract by the organism. |
Gram stained smear and culture.
Blood agar: β-Hemolytic colonies that are resistant to bacitracin. Organisms hydrolyze hippurate and are CAMP test positive. |
Penicillin G.
No vaccine. Penicillin or ampicillin should be given to mothers if prolonged rupture of membranes occurs, if mother has a fever, or if the neonate is premature. |
| Enterococcus faecalis | Gram positive cocci in chains.
Catalase negative. |
Urinary tract and biliary tract infections are most frequent.
Endocarditis is rare but life threatening. |
Human colon; urethra and female genital tract can be colonized.
May enter bloodstream during gastrointestinal (GI) or genitourinary tract procedures. May infect other sites (e.g., endocarditis). |
Pyogenic organism.
No exotoxins identified. |
Gram stained smear and culture. αor Blood agar: β-Hemolytic or nonhemolytic colonies Grows in 6.5% NaCl and hydrolyzes esculin in the presence of 40% bile. Serologic tests not useful. |
Penicillin or vancomycin plus an aminoglycoside.
Organism is resistant to either drug given individually, but given together, they have a synergistic effect. Aminoglycoside alone is ineffective because it cannot penetrate. Penicillin or vancomycin weakens the cell wall, allowing the aminoglycoside to penetrate. Vancomycin resistant enterococci (VRE) are important causes of nosocomial infections. Linezolid can be used to treat VRE. Penicillin and gentamicin should be given to patients with damaged heart valves prior to intestinal or urinary tract procedures. No vaccine is available. |
| Streptococcus pneumoniae (Pneumococcus) | Gram positive “lancet shaped” cocci in pairs (diplococci) or short chains. α Hemolytic colonies.
Catalase negative. Growth is inhibited by optochin in contrast to viridans streptococci, which are resistant. Colonies are bile-soluble. One of the three classical encapsulated pyogenic bacteria (Neisseria |
Pneumonia and meningitis in adults
Otitis media and sinusitis in children. |
Human upper respiratory tract.
Transmission is via respiratory droplets. |
Induces pyogenic inflammatory response. No known exotoxins. Polysaccharide capsule retards phagocytosis.
Antipolysaccharide antibody opsonizes the organism and provides type-specific immunity. IgA protease degrades secretory IgA on respiratory mucosa, allowing colonization. Viral respiratory infection predisposes to pneumococcal pneumonia by damaging mucociliary elevator; splenectomy predisposes to sepsis. Skull fracture with spinal fluid leakage from nose predisposes to meningitis. |
Gram stained smear and culture.
Blood agar: α- Hemolytic colonies. Growth inhibited by bile and optochin. Quellung reaction occurs (swelling of capsule with type-specific Serologic tests for antibody not useful. Tests for capsular antigen in spinal fluid and C polysaccharide in urine can be diagnostic. |
Penicillin G.
Low-level and high-level resistance to penicillin is caused by alterations in penicillin binding proteins. No β-lactamase is made. Two vaccines are available. Oral penicillin is used in immunocompromised children. |
| Viridans Group Streptococci (e.g., Streptococcus sanguis, Streptococcus mutans) | Gram-positive cocci in chains. αHemolytic colonies.
Catalase negative. Growth is resistant to optochin in contrast to pneumococci, which are inhibited. Colonies are not dissolved by bile. |
Endocarditis is the most important disease. Also brain abscess, especially in mixed infections with mouth anaerobes. S. mutans implicated in dental caries. |
Habitat is the human oropharynx. Organism enters bloodstream during dental procedures. | Low virulence organism. Bacteremia from dental procedures spreads organism to damaged heart valves.
Organism is protected No known toxins. Glycocalyx composed of polysaccharide enhances adhesion to heart valves. |
Gram stained smear and culture.
Blood agar: α- Hemolytic colonies. Growth not inhibited by bile or optochin, in contrast to pneumococci. Serologic tests not useful. |
Penicillin G with or without an aminoglycoside.
Penicillin to prevent endocarditis in patients with damaged or prosthetic heart valves who undergo dental procedures. |
| Gram negative cocci | ||||||
| Neisseria meningitidis (Meningococcus) | Gram negative “kidney bean” diplococci.
Oxidase positive. Large polysaccharide capsule. One of the three classic encapsulated pyogenic bacteria (S. pneumoniae and H. influenzae are the other two). |
Meningitis and meningococcemia. | Human upper respiratory tract; transmission is via respiratory droplets. | After colonizing the upper respiratory tract, the organism reaches the meninges via the bloodstream.
Endotoxin in cell wall causes symptoms of septic shock seen in meningococcemia. No known exotoxins; IgA protease produced. Polysaccharide capsule is antiphagocytic and is the main virulence factor of this aggressive pathogen. Deficiency in late complement components predisposes to recurrent meningococcal infections. |
Gram stained smear and culture.
Chocolate agar: Oxidase positive colonies. Ferments maltose in contrast to gonococci, which do not. |
Penicillin G (no significant resistance).
The vaccines against groups A, C, Y, and W135 meningococci contain the polysaccharide capsule as the immunogen. The vaccine against group B meningococci contains factor H-binding protein as the immunogen. The polysaccharide vaccine exists in two forms: the conjugate Rifampin or ciprofloxacin is given to close contacts to decrease oropharyngeal carriage. |
| Neisseria gonorrhoeae (Gonococcus) | Gram negative “kidney bean” diplococci.
Oxidase positive. |
Gonorrhea.
Neonatal conjunctivitis and pelvic inflammatory disease. |
Human genital tract.
Transmission in adults is by sexual contact. Transmission to neonates is during |
Organism invades mucous membranes and causes inflammation.
Endotoxin present but weaker than that of meningococcus, so less severe disease when bacteremia occurs. No exotoxins identified. IgA protease and pili are virulence factors. |
Gram stained smear and culture.
Organism visible intracellularly within neutrophils in urethral exudate. Thayer-Martin medium: Oxidase positive colonies Gonococci do not ferment maltose, whereas meningococci do. S Nucleic acid amplification tests (NAATs) are used as a screening test in urogenital infections. |
Ceftriaxone for uncomplicated cases.
Doxycycline or azithromycin for urethritis caused by coinfection with Chlamydia trachomatis. High/low-level resistance to penicillin is caused by penicillinase or reduced permeability and altered binding proteins. No vaccine. Condoms offer protection. Treat eyes of newborns with erythromycin ointment or silver nitrate to prevent conjunctivitis. |
| Gram positive rods | ||||||
| Bacillus anthracis | Aerobic, gram positive, Spore forming rods. Capsule composed of poly-D-glutamate. B. anthracis is the only medically important organism that has a capsule composed of amino acids rather than polysaccharides.
|
Anthrax. | Soil.
Transmission is by contact with infected animals or inhalation of spores from animal hair and wool. |
Anthrax toxin consists of three proteins: edema factor, which is an adenylate cyclase; lethal factor, which kills cells by inhibiting a signal transduction protein involved in cell division; protective antigen, which mediates the entry of the other two components into the cell. The capsule is antiphagocytic. |
Gram stained
Blood agar: aerobic culture B. anthracis is non-motile, in contrast to other Bacillus species. Rise in antibody titer in indirect hemagglutination test is diagnostic. |
Penicillin G (no significant resistance).
Vaccine consisting of protective antigen is given to individuals in high risk occupations. |
| Bacillus cereus | Aerobic, gram positive, Spore forming rod. |
Gastroenteritis (food poisoning). | Grains, such as rice.
Spores survive boiling during preparation of rice, then germinate when rice is held at warm temperature.
|
Two enterotoxins are produced: 1) one acts like cholera toxin (i.e., cyclic adenosine monophosphate [AMP] is increased within enterocytes); 2) the other acts like staphylococcal enterotoxin (i.e., it is a superantigen). |
None. | Symptomatic only. No vaccine. |
| Clostridium tetani | Anaerobic, gram positive Spore forming rods. Spore is at one end (“terminal spore”), organism looks like a tennis racket. |
Tetanus. | Soil.
Organism enters through breaks in the skin. |
Spores germinate under anaerobic conditions in the wound.
Organism produces exotoxin, which blocks release of inhibitory neurotransmitters (glycine and γaminobutyric acid [GABA]) from spinal neurons. Excitatory neurons are unopposed, and extreme muscle spasm (tetanus, spastic paralysis) results. Tetanus toxin (tetanospasmin) is a protease that cleaves proteins involved in the release of the inhibitory neurotransmitters. |
Primarily a clinical diagnosis.
Rarely isolated. |
Hyperimmune human globulin to neutralize toxin.
Also penicillin G and diazepam. No significant resistance to penicillin. Toxoid vaccine, usually given to children in combination with diphtheria toxoid and acellular pertussis vaccine (DTaP). Give tetanus toxoid booster every 10 years. |
| Clostridium botulinum | Anaerobic, gram positive | Botulism. | Soil.
Organism and botulinum toxin transmitted in improperly preserved food. |
Botulinum toxin is a protease that cleaves proteins involved in the release of acetylcholine at the myoneural junction, causing flaccid paralysis.
Failure to sterilize food during preservation allows spores to survive. Spores germinate in anaerobic environment and produce toxin. The toxin is heat labile; foods eaten without proper cooking are usually implicated. |
Detection of toxin involves either antitoxin in serologic tests or production of the disease in mice. |
Antitoxin to types A, B, and E made in horses.
Respiratory support may be required. |
| Clostridium perfringens | Anaerobic, gram positive Spore forming rods. |
Gas gangrene (necrotizing fasciitis, myonecrosis) and food poisoning. | Soil and human colon.
Myonecrosis results from contamination of wound with soil or feces. Food poisoning --> ingestion of contaminated food. |
Gas gangrene in wounds is caused by germination of spores under anaerobic conditions and the production of cytotoxic factors: alpha toxin, a lecithinase that cleaves cell membranes.
Gas in tissue (CO2 and H2) is produced by organism’s anaerobic metabolism. Food poisoning is caused by production of enterotoxin; acts as a superantigen, similar to that of S. aureus. |
Gram stained smear plus anaerobic culture.
Production of lecithinase is detected on egg yolk agar and identified by enzyme inhibition with specific antiserum. |
Penicillin G plus debridement of the wound in gas gangrene/and to prevent.
Only symptomatic treatment needed in food poisoning. No vaccine. |
| Clostridium difficile | Anaerobic, gram positive, Spore forming rods. |
Pseudomembranous colitis. | Human colon.
Transmission is fecal–oral. |
Antibiotics suppress normal flora of colon, allowing C. difficile to overgrow and produce large amounts of exotoxins.
Exotoxins A and B inhibit GTPases, causing inhibition of signal transduction and depolymerization of actin filaments. --> Apoptosis and death of enterocytes. |
Exotoxin in the stool is typically detected by ELISA test or PCR assay.
Exotoxin in stool can also be detected by cytopathic effect on cultured cells. A screening test that detects the glutamine dehydrogenase of the organism in stool is available. |
Oral vancomycin or fidaxomicin can be used.
If life threatening, use vancomycin plus metronidazole. No vaccine. |
| Corynebacterium diphtheriae | Club-shaped gram positive Rods arranged in V or L shape. Granules stain metachromatically. Aerobic, non–spore forming |
Diphtheria. | Human throat.
Transmission is via respiratory droplets. |
Organism secretes an exotoxin that inhibits protein synthesis by adding ADPribose to elongation factor2 (EF2). Toxin has two components: Pseudomembrane in throat caused by death of mucosal epithelial cells. |
Gram stained smear and culture.
Tellurite plate: Black colonies Document toxin production with precipitin test or by disease produced in laboratory animals. |
Antitoxin made in horses neutralizes the toxin.
Penicillin G kills the organism. Toxoid vaccine (toxoid is formaldehyde treated toxin), usually given to children in combination with tetanus toxoid and acellular |
| Listeria monocytogenes | Small gram positive rods.
Aerobic, non–spore forming |
Meningitis and sepsis in newborns and immunocompromised adults.
Gastroenteritis. |
Organism colonizes the GI and female genital tracts; in nature, it is widespread in animals, plants, and soil.
Transmission is across the placenta or by contact during delivery. Outbreaks of sepsis in neonates and gastroenteritis in the general population are related to ingestion of unpasteurized milk products. |
Listeriolysin is an exotoxin that degrades cell membranes.
Reduced cell mediated immunity and immunologic immaturity as in neonates predispose to disease. Intracellular pathogen that moves from cell to cell via “actin rockets.” |
Gram stained smear and culture.
Blood agar: β-Hemolytic colonies; small Tumbling motility. |
Ampicillin with or without gentamicin.
Pregnant women and immunocompromised patients should not ingest unpasteurized milk products or raw vegetables. Trimethoprim/sulfamethoxazole given to immunocompromised patients to prevent Pneumocystis pneumonia can also prevent listeriosis. No vaccine |
| Gram negative rods related to the enteric tract | ||||||
| Escherichia coli | Facultative gram negative rods
Ferments lactose. |
Urinary tract infection (UTI), sepsis, neonatal meningitis, and “traveler’s diarrhea”
Hemolyticuremic |
Human colon, vagina and urethra.
Acquired during birth in neonatal meningitis and by the fecal–oral route in diarrhea. |
Endotoxin in cell wall causes septic shock.
Heat labile toxin (LT) stimulates adenylate cyclase by ADPribosylation --> Increased cyclic AMP causes outflow of chloride ions and water, resulting in diarrhea. Heat stable toxin (ST) causes diarrhea, perhaps by stimulating guanylate cyclase. Virulence factors include pili for attachment to mucosal surfaces and a capsule that impedes phagocytosis. Shiga toxin (verotoxin) is an enterotoxin produced by E. coli strains O157:H7 serotype. It inhibits protein synthesis by removing adenine from the 28S rRNA of human ribosomes. Predisposing factors to UTI in women include the proximity of the anus to the vagina and urethra. Abnormalities (e.g., strictures, valves, and stones) Indwelling urinary catheters and intravenous lines predispose to UTI and sepsis, respectively. Colonization of the vagina leads to neonatal meningitis The main virulence factor for neonatal meningitis is the K1 capsular polysaccharide. |
Gram stained smear and culture.
Eosin–methylene blue (EMB) or MacConkey’s agar: Lactose fermenting Triple sugar iron (TSI) agar: Acid slant and acid butt with gas but no H2S. Differentiate from other lactose positive organisms by biochemical reactions. For epidemiologic studies, type organism by O and H antigens by using known antisera. |
Ampicillin or sulfonamides for UTIs. Third generation cephalosporins for meningitis and sepsis.
Rehydration is effective in traveler’s diarrhea; trimethoprimsulfamethoxazole may shorten duration of symptoms. Antibiotic resistance mediated by plasmid encoded enzymes (e.g., β-lactamase and aminoglycoside modifying enzymes). Prevention of UTI involves limiting the frequency and duration of urinary catheterization. Prevention of sepsis involves promptly removing or switching sites of intravenous lines. Traveler’s diarrhea is prevented by eating only cooked food and drinking boiled water in certain countries. Prophylactic doxycycline or bismuth subsalicylate may prevent traveler’s diarrhea. |
| Salmonella typhi | Facultative gram negative rods.
Non–lactose fermenting. Produces H2S. |
Typhoid fever. | Human colon only.
Transmission is by the fecal–oral route. |
Infects the cells of the reticuloendothelial system, especially in the liver and spleen.
Endotoxin in cell wall causes fever. Capsule (Vi antigen) is a virulence factor. No exotoxins known. Decreased stomach acid resulting from ingestion of antacids or gastrectomy predisposes to Salmonella infections. Chronic carrier state established in gallbladder. Organism excreted in bile results in fecal–oral spread to others. |
Gram stained smear and culture.
EMB or MacConkey’s agar: Non–lactose fermenting TSI agar: Alkaline slant and acid butt, with no gas and a small amount of H2S. Biochemical and serologic reactions used to identify species. Identity can be determined by using known antisera against O, H, and Vi antigens in agglutination test. Widal test detects agglutinating antibodies to O and H |
Most effective drug is ceftriaxone. Ampicillin and trimethoprim/sulfamethoxazole can be used in patients who are not severely ill.
Resistance to chloramphenicol and ampicillin is mediated by plasmid encoded acetylating enzymes and βlactamase, Public health measures. Two vaccines are in common use; one vaccine contains purified Vi polysaccharide capsule as the immunogen and the other contains live, attenuated S. typhi as the immunogen. |
| Salmonella enterica (often called Salmonella enteritidis) | Facultative gram negative rods.
Non–lactose fermenting. Produces H2S Motile, in contrast to Shigella. |
Enterocolitis.
Sepsis with metastatic abscesses occasionally. |
Enteric tract of humans and animals (e.g., poultry and domestic livestock). Fecal–oral route - transmission | Invades the mucosa of the small and large intestines.
Can enter blood, causing sepsis. Infectious dose is at least 100,000 organism is inactivated by stomach acid. Endotoxin in cell wall; no exotoxin. Predisposing factors include lowered stomach acidity from either antacids or gastrectomy. Sickle cell anemia predisposes to Salmonella osteomyelitis. |
Gram stained smear and culture.
EMB or MacConkey’s agar: Non–lactose fermenting TSI agar: Alkaline slant and acid butt, with gas and H2S. Biochemical and serologic reactions used to identify species. Can identify the organism by using known antisera in agglutination assay. Widal test detects antibodies in patient’s serum to the O and H antigens of the organism but is not widely used. |
Antibiotics usually not recommended for uncomplicated enterocolitis.
Ceftriaxone or other drugs are used for sepsis, depending on organisms, much greater than the infectious dose of Shigella. Infectious dose is high because sensitivity tests. Resistance to ampicillin and chloramphenicol is mediated by plasmid encoded β-lactamases and acetylating enzymes, respectively. Public health measures. Do not eat raw eggs or meat. No vaccine is available. |
| Shigella species (e.g., Shigella dysenteriae, Shigella sonnei) | Facultative gram negative rods.
Non–lactose fermenting. Non-motile, in contrast to Salmonella. |
Enterocolitis (dysentery) | Human colon only; no animal carriers for Shigella.
Transmission is by the fecal–oral route. |
Invades the mucosa of the ileum and colon but does not penetrate farther; therefore, sepsis is rare.
Endotoxin in cell wall. Infectious dose is much lower (1–10 organisms) than that of Salmonella because it is resistant to stomach acid. Children in psychiatric hospitals and day care centers experience outbreaks of shigellosis. No chronic carrier state. |
Gram stained smear and culture.
EMB or MacConkey’s agar: Non–lactose fermenting TSI agar: Alkaline slant with an acid butt and no gas or H2S. Identified by biochemical reactions or by serology with antiO |
Fluid and electrolyte replacement only.
In severe cases, ciprofloxacin. Resistance is mediated by plasmid encoded enzymes (e.g., βlactamase, which degrades ampicillin, and a mutant pteroate synthetase, which reduces sensitivity to sulfonamides). Public health measures. |
| Vibrio cholerae | Comma shaped gram negative rods
Oxidasepositive (distinguishes them from Enterobacteriaceae) |
Cholera | Human colon and shellfish.
Transmission is by the fecal–oral route. |
Massive, watery diarrhea caused by enterotoxin that activates adenylate cyclase by adding ADPribose to the stimulatory G protein.
Increase in cyclic AMP activates cyclic AMP dependent kinase that phosphorylates a membrane ion channel. This causes an outflow of chloride ions and water. Toxin has two components: Organism produces mucinase, which enhances attachment to the intestinal mucosa. Role of endotoxin is unclear. |
Gram stained smear and culture.
Agglutination of the isolate with known antisera confirms the identification. |
Fluid and electrolyte replacement.
Tetracycline is not necessary but shortens duration. Public health measures. For travelers to endemic areas, oral vaccine containing live, attenuated bacteria is available in United States. Tetracycline used for close contacts. |
| Campylobacter jejuni | Comma shaped gram negative rods
Microaerophilic Grows well at 42°C |
Enterocolitis | Human and animal feces.
Transmission is by the fecal–oral route. |
Invades mucosa of the colon but does not penetrate.
No enterotoxin known. |
Gram stained smear
Culture on special agar (e.g., Skirrow’s agar) at 42°C in high CO2, low O2 |
Usually symptomatic treatment only; erythromycin for severe disease.
Public health measures. |
| Helicobacter pylori | Curved gram negative rod | Gastritis and peptic ulcer.
Risk factor for gastric carcinoma. |
Human stomach.
Transmission is by ingestion. |
Organisms synthesize urease, which produces ammonia that damages gastric mucosa.
Ammonia also neutralizes acid pH in stomach, which allows the organism to live in gastric mucosa. |
Gram stain and culture.
Urease positive. Serologic tests for antibody and the “urea breath” test are useful. |
One regimen is amoxicillin, clarithromycin, metronidazole, and a proton pump inhibitor such as omeprazole.
No vaccine. |
| Klebsiella pneumoniae | Facultative gram negative rods
Large polysaccharide capsule |
Pneumonia, UTI, and sepsis | Human upper respiratory and enteric tracts.
Organism is transmitted to the lungs by aspiration from upper respiratory tract and by inhalation of respiratory droplets. It is transmitted to the urinary tract by ascending spread of fecal flora. |
Endotoxin causes fever and shock associated with sepsis.
No exotoxin known. Organism has large capsule, which impedes phagocytosis. Chronic pulmonary disease predisposes to pneumonia; catheterization predisposes to UTI. |
Gram stained smear and culture.
MacConkey’s agar: Lactose fermenting Characteristic mucoid colonies due to abundant polysaccharide capsule. |
Cephalosporins alone or with aminoglycosides, but antibiotic sensitivity testing must be done.
Resistance is mediated by plasmid encoded No vaccine. |
| Enterobacter cloacae | Enteric gram negative rod
Similar to K. pneumoniae. |
Causes hospital- acquired pneumonia, UTI, and sepsis. | Highly antibiotic resistant. | |||
| Serratia marcescens | Enteric gram negative rod
Similar to K. pneumoniae. |
Causes hospital-acquired pneumonia, UTI, and sepsis. | Red pigmented colonies. | Highly antibiotic resistant. | ||
| Proteus species (e.g., Proteus vulgaris, Proteus mirabilis) | Facultative gram negative rods.
Non–lactose fermenting. Highly motile. Produce urease. Antigens of OX strains of P. vulgaris cross-react with many rickettsiae. |
UTI and sepsis. | Human colon and the environment (soil and water).
Transmission to urinary tract is by ascending |
Endotoxin causes fever and shock associated with sepsis.
No exotoxins known. Urease is a virulence factor because it degrades urea to produce ammonia, which raises the pH. This leads to “struvite” stones, which can obstruct urine flow, damage urinary epithelium, and serve as a nidus for recurrent infection by trapping bacteria within the stone. Predisposing factors: colonization of the vagina, urinary catheters, and abnormalities of the urinary tract such as strictures, valves, and stones. |
Gram stained smear and culture.
Blood Agar: “Swarming” (spreading) as a consequence of the high motility. EMB or MacConkey’s agar: Non–lactose fermenting TSI agar: Alkaline slant and acid butt with H2S. Organism produces urease, whereas Salmonella, which can appear similar on TSI agar, does not. Note: P. mirabilis is indole negative, whereas P. vulgaris, M. morganii, and Providencia species are indole positive. |
Trimethoprim/sulfamethoxazole nitrofurantoin, ciprofloxacin for uncomplicated UTIs
Third generation cephalosporin used for serious infections. P. mirabilis is more likely to be sensitive to antibiotics such as ampicillin Antibiotic sensitivities should be tested. Resistance is mediated by plasmid encoded enzymes. No vaccine. Prompt removal of urinary catheters helps prevent UTIs. |
| Pseudomonas aeruginosa | Aerobic gram negative rods.
Non–lactose fermenting Pyocyanin (blue-green) pigment produced Oxidase positive |
Wound infection, UTI, pneumonia, and sepsis.
Nosocomial infections, in burn patients and those with cystic fibrosis. Causes endocarditis in intravenous drug users. |
Water sources (e.g., in hospital respirators and humidifiers).
Also inhabits the skin, upper respiratory tract, and colon of about 10% of people. Transmission is via water aerosols, aspiration, and fecal contamination. |
Endotoxin is responsible for fever and shock associated with sepsis.
Exotoxin A, which acts like diphtheria toxin (inactivates EF2). Pili and capsule are virulence factors that mediate attachment and inhibit phagocytosis, respectively. Glycocalyx producing strains predominate in chronic infections in cystic fibrosis patients. Severe burns and neutropenia are important predisposing factors. |
Gram stained smear and culture.
EMB or MacConkey’s agar: Non–lactose fermenting TSI agar: Alkaline slant and alkaline butt because the sugars are not fermented. Oxidase positive. |
Antibiotics chosen on the basis of antibiotic sensitivities; resistance is common.
Antipseudomonal penicillin and Resistance is mediated by a variety of plasmid encoded enzymes (e.g., βlactamases Disinfection of water related equipment in the hospital, handwashing, and prompt removal of urinary and intravenous catheters. No vaccine. |
| Gram negative rods related to the respiratory tract | ||||||
| Haemophilus influenzae | Small gram negative (coccobacillary) rods.
Requires factors X (hemin) and V (NAD) for growth. Of the six capsular polysaccharide types, type b causes 95% of invasive disease. Type b capsule is polyribitol phosphate. |
Sinusitis, otitis media, and pneumonia are common.
Epiglottitis is uncommon, but H. influenzae is the most important cause. H. influenzae used to be a leading cause of meningitis, but the vaccine has greatly reduced the number of cases. |
Upper respiratory tract.
Transmission is via respiratory droplets. |
Polysaccharide capsule is the most important determinant of virulence.
Unencapsulated (“untypeable”) strains cause mucosal infections but not invasive infections. IgA protease is produced. Immune response of the child to capsular polysaccharides can be inadequate. No exotoxins identified. |
Gram stained smear
Chocolate agar: Growth requires both factors X and V. Determine serotype by using antiserum in various tests (e.g., latex agglutination). Capsular antigen can be detected in serum or cerebrospinal fluid. |
Ceftriaxone for meningitis.
Approximately 25% of strains produce β-lactamase. Vaccine containing the type b capsular polysaccharide conjugated to diphtheria toxoid or other protein is given between 2 and 18 months of age. Rifampin can prevent meningitis in close contacts. |
| Bordetella pertussis | Small gram negative rods. | Whooping cough (pertussis). | Human respiratory tract.
Transmission is via respiratory droplets. |
Pertussis toxin stimulates adenylate cyclase by adding ADPribose onto the inhibitory G protein.
Toxin has two components: In addition, extracellular adenylate cyclase is produced, which can inhibit killing by phagocytes. Tracheal cytotoxin damages ciliated epithelium of respiratory tract. |
Gram stained smear plus culture on BordetGengou agar.
Identified by biochemical reactions and slide agglutination with known antisera. PCR tests, if available, are both sensitive and specific. |
Azithromycin.
The acellular vaccine containing pertussis toxoid and four other purified proteins is recommended rather than the killed vaccine, Usually given to children in combination with diphtheria and tetanus toxoids (DTaP). Azithromycin is useful in unimmunized people who are known to be exposed. |
| Legionella pneumophila | Gram negative rods, but stain poorly with standard Gram stain.
Require increased iron and cysteine for growth in culture. Sixteen serogroups; most cases caused by serogroup 1. |
Legionnaires’ disease (“atypical” pneumonia). | Environmental water sources.
Transmission is via aerosol from the water source. Person-to-person transmission does not occur. |
Aside from endotoxin, no toxins, enzymes, or virulence factors are known.
Predisposing factors include being older than 55 years, smoking, and having a high alcohol intake. Immunosuppressed patients are highly susceptible. The organism replicates intracellularly; therefore, cell mediated immunity is an important host defense. Smoking damages alveolar macrophages, it predisposes to pneumonia. |
Microscopy with silver impregnation stain or fluorescent antibody.
Culture on charcoal yeast extract agar containing increased amounts of iron and cysteine. Urinary antigen provides rapid diagnosis for serogroup 1 bacteria only. Diagnosis can be made serologically by detecting rise in antibody titer in patient’s serum. |
Azithromycin or erythromycin. Rifampin can be added in severe cases.
No vaccine or prophylactic. |
| Pasteurella multocida | Small gram negative rods. | Wound infection (e.g., cellulitis). | Reservoir is the mouth of many animals, especially cats and dogs.
Transmission is by animal bites. |
Spreads rapidly in skin and subcutaneous tissue.
No exotoxins. |
Gram stained smear and culture. | Penicillin G.
amoxicillin-clavulanate should be given to individuals with cat bites. No vaccine. |
| Bartonella henselae | Small gram negative rod. | Cat Scratch disease (CSD) and bacillary angiomatosis (BA). | Reservoir is the cat’s mouth and transmitted by scratch or bite. | Low virulence organism.
CSD is self limited in immunocompetent individuals, but BA occurs in immunocompromised individuals. |
Diagnosis of CSD made by serologic tests. Biopsy of BA lesion shows pleomorphic rods using Warthin-Starry stain. |
None for CSD.
Doxycycline or erythromycin for BA. No vaccine. |
| Mycobacteria | ||||||
| Mycobacterium tuberculosis | Aerobic, acid fast rods.
High lipid content of cell wall, which prevents dyes used in Gram stain from staining organism. Lipids include mycolic acids and wax D. Grows very slowly, which requires that drugs be present for long periods (months). Catalase positive, which is required to activate isoniazid to the active drug. |
Tuberculosis. | Human lungs.
Transmission is via respiratory droplets produced by coughing. |
Granulomas and caseation mediated by cellular immunity (i.e., macrophages and CD4 positive T-cells [delayed hypersensitivity]).
Cord factor (trehalose mycolate) correlates with virulence. No exotoxins or endotoxin. Suppression of cell mediated immunity increases risk of reactivation and dissemination. |
Acid fast rods seen with Ziehl-Neelsen (or Kinyoun) stain.
Slow growing (3–6 weeks) colony on Löwenstein- Jensen medium. Organisms produce niacin and are catalase positive. Skin Test—Purified protein derivative (PPD) skin test is positive if induration measuring 10 mm or more appears 48 hours after inoculation. Induration is caused by a delayed hypersensitivity response. Positive skin test indicates that the person has been infected but not necessarily that the person has the disease tuberculosis. |
Long-term therapy (6–9 months) with three drugs: isoniazid, rifampin, and pyrazinamide. A fourth drug, ethambutol, is used in severe cases, in immunocompromised patients (e.g., those with acquired immunodeficiency syndrome [AIDS]), and where the chance of isoniazid resistant organisms is high, as in Southeast Asians.
Most patients become noninfectious within 2 weeks of adequate therapy. Treatment of latent (asymptomatic) infections consists of isoniazid taken for 6 to 9 months or isoniazid plus rifampin for 3 months. Multidrug-resistant (MDR) strains have emerged and require other drug combinations. Vaccine used rarely in the United States but widely used in parts of Europe and Asia. |
| Actinomycetes | ||||||
| Actinomyces israelii | Anaerobic, gram positive filamentous, branching rods | Actinomycosis (abscesses with draining sinus tracts) | Human mouth, especially anaerobic crevices around the teeth.
Transmission into tissues occurs during dental disease or trauma. Organism also aspirated into lungs, causing thoracic actinomycosis. Retained intrauterine device (IUD) predisposes to pelvic actinomycosis. |
No toxins or virulence factors known.
Organism forms sinus tracts that open onto skin and contain “sulfur granules,” mats of intertwined filaments of bacteria. |
Gram stained smear
Blood agar plate: anaerobic “Sulfur granules” visible in the pus. |
Penicillin G and surgical drainage.
No vaccine available. |
| Nocardia asteroides | Aerobic, gram positive filamentous, branching rods.
Weakly acidfast. |
Nocardiosis (especially lung and brain abscesses). | Soil.
Transmission is via airborne particles |
No toxins or virulence factors known.
Immunosuppression and cancer predispose to infection. |
Gram stained smear and modified ZiehlNeelsen stain.
Blood agar plate: anaerobic No serologic tests. |
Sulfonamides.
No vaccine available. |
| Mycoplasmas | ||||||
| Mycoplasma pneumoniae | Smallest free living organisms. Not seen on Gram stained smear--> no cell wall, so dyes are not retained.
Penicillins and cephalosporins are not effective because there is no cell wall (peptidoglycan). The only bacteria with cholesterol in cell membrane. |
“Atypical” pneumonia. | Human respiratory tract.
Transmission is via respiratory droplets. |
No exotoxins | Gram stain not useful.
Can be cultured on special bacteriologic media but takes at least 10 days to grow. Positive cold–agglutinin test is presumptive evidence. Complement fixation test for antibodies to Mycoplasma pneumoniae is more specific. |
Azithromycin or doxycycline.
No vaccine available. |
| Spirochetes | ||||||
| Treponema pallidum | Spirochetes.
Not seen on Gram stained smear because organism is too thin. Not cultured in vitro. |
Syphilis. | Human genital tract.
Transmission is by sexual contact and from mother to fetus across the placenta. |
Organism multiplies at site of inoculation and then spreads widely via the bloodstream.
Many features of syphilis are attributed to blood vessel involvement causing vasculitis. Primary and secondary lesions heal spontaneously. Tertiary lesions consist of gummas (granulomas in bone, muscle, and skin), aortitis, or central nervous system inflammation. No toxins or virulence factors known. |
Seen by darkfield microscopy or immunofluorescence.
Serologic tests important: VDRL and RPR are nontreponemal Antigen in VDRL and RPR is beef heart cardiolipin; antigen in FTAABS VDRL declines with treatment, whereas FTAABS remains positive for life. |
Penicillin is effective in the treatment of all stages of syphilis.
In primary and secondary syphilis, use benzathine penicillin G (a depot preparation) because T. pallidum grows slowly, so drug must be present for a long time. There is no resistance. Benzathine penicillin given to contacts. No vaccine available. |
| Borrelia burgdorferi | Spirochetes.
Gram stain not useful. Can be cultured in vitro but not usually done. |
Lyme disease. | The main reservoir is the white-footed mouse. Very small nymph stage of ixodid tick (deer tick) is the most common vector.
Eighty percent of cases are in the northeastern states of Connecticut, New York, and New Jersey. Very small nymph stage of ixodid tick (deer tick) is the most common vector. Tick must feed on person for at least 24 hours to deliver an infectious dose of B. burgdorferi. |
Organism invades skin, causing a rash called erythema migrans.
It then spreads via the bloodstream to involve primarily the heart, joints, and central nervous system. No toxins or virulence factors identified. |
Diagnosis is usually made serologically (i.e., by detecting IgM antibody).
Confirm positive serologic test with Western blot assay. |
Doxycycline for early stages; penicillin G for late stages.
No vaccine available. Avoid tick bite. Can give doxycycline or amoxicillin to people who are bitten by a tick in endemic areas. |
| Chlamydiae | ||||||
| Chlamydia trachomatis | Obligate intracellular parasites.
Not seen on Gram stained smear. Exists as inactive elementary body extracellularly and as metabolically active, dividing reticulate body intracellularly. |
Nongonococcal urethritis, cervicitis, inclusion conjunctivitis, lymphogranuloma venereum, and trachoma.
Also pneumonia in infants. |
Human genital tract and eyes.
Transmission is by sexual contact and during passage of neonate Transmission in trachoma is chiefly by hand to eye contact. |
No toxins or virulence factors known. | Nucleic acid amplification test (NAAT) using the patient’s urine is used to diagnose chlamydial sexually transmitted disease.
Gram stain of urethral exudates that show neutrophils but no gram negative diplococci (gonococci) is presumptive evidence for Cytoplasmic inclusions seen on Giemsa stained or fluorescent antibody–stained smear of exudate. Organism grows in cell culture and embryonated eggs, but these are not often used. |
A tetracycline (e.g., doxycycline) or a macrolide (e.g., azithromycin).
Erythromycin effective in infected mother to prevent neonatal disease. No vaccine available. |
| Chlamydia pneumoniae | Same as C. trachomatis. | Atypical pneumonia. | Human respiratory tract.
Transmission is by respiratory aerosol. |
No toxins or virulence factors known. | NAAT and serologic tests for antibody in patient’s serum. | A tetracycline, such as doxycycline.
No vaccine available. |
| Rickettsiae | ||||||
| Rickettsia rickettsii | Obligate intracellular parasites.
Not seen well on Gram stained smear. Antigens cross-react with OX strains of P. vulgaris (WeilFelix reaction). |
Rocky Mountain spotted fever. | Dermacentor (dog) ticks are both the vector and the main reservoir.
Transmission is via tick bite. Dogs and rodents can be reservoirs as well. |
Organism invades endothelial lining of capillaries, causing vasculitis.
No toxins or virulence factors identified. |
Diagnosis made by detecting antibody in serologic tests such as the ELISA test.
Weil-Felix test is no longer used. Stain and culture rarely done. |
Doxycycline.
Protective clothing and prompt removal of ticks. Tetracycline effective in exposed persons. No vaccine available. |
Source: Adapted under Fair Use from Table 5-1 Classification of Medically Important Bacteria. In: Levinson W, Chin-Hong P, Joyce EA, Nussbaum J, Schwartz B. eds. Review of Medical Microbiology & Immunology: A Guide to Clinical Infectious Diseases, 16e. McGraw Hill; 2020. Accessed January 06, 2025. https://accessmedicine.mhmedical.com/content.aspx?bookid=2867§ionid=242765525
